The preoptic area has long been identified as a region that is important for induction of sleep. It contains populations of sleep-active neurons, and lesion in this area cause long-lasting insomnia. However, the circuitry that is critical for inducing sleep has never been identified. We recently identified as specific cell group, the VLPO, that contains neurons that show activation, as monitored by expression of the immediate early gene (IEG) product Fos, specifically during sleep. We hypothesize that these sleep-related neurons in the VLPO innervate the main components of the ascending arousal system in the brainstem, providing profound inhibition that allows the onset of sleep. We plan to test this hypothesis with four sets of experiments: (1) We will trace the efferent connections of the VLPO using a combination of anterograde tracers and double retrograde fluorescent tracing with immunocytochemistry, to determine whether sleep-active neurons in the VLPO innervate monoaminergic and cholinergic brainstem cel groups, whether these descending projections are collateralized or arise from separate neuronal populations, and whether the neurons participating in this projection use GABA and galanin as neurotransmitters, (2) We will trace the afferents to the VLPO and the neurotransmitters they employ, to determine the inputs that drive the sleep-active neurons, (3) We will examine the effects of unilateral lesions of the VLPO on ipsilateral EEG delta (0.1-4 Hz) power. Finally, (4) we will determine whether unilateral stimulation of the VLPO with endogenous sleep-inducing substances, including prostaglandin D2, adenosine, delta sleep-inducing peptide, or a sleep-inducing lipid, produces ipsilateral EEG slowing and VLPO Fos expression. These experiments will provide critical evidence to determine whether the VLPO has the appropriate connections and physiological responses to support a major role in inducing sleep.